In the following excerpt, Swami Sivananda gives the answer on the virtues of semen retention:
My dear brothers! The vital energy, the Veerya that supports your life, which is the Prana of Pranas, which shines in your sparkling eyes, which beams in your shining cheeks, is a great treasure for you. Remember this point well. Veerya is the quintessence of blood. One drop of semen is manufactured out of forty drops of blood. Mark here how valuable this fluid is!
A tree draws the essence or Rasa from the earth. This essence is circulated throughout the tree, its twigs, branches, leaves, flowers and fruits. The shining colours and life in the leaves, flowers and fruits are due to this Rasa. Similarly, the Veerya that is manufactured by the cells of the testes out of blood gives colour and vitality to the human body and its different organs.
According to Ayurveda, semen is the last Dhatu that is formed out of food. Out of food is manufactured chyle. Out of chyle comes blood. Out of blood comes flesh. Out of flesh comes fat. Out of fat comes bone. Out of bone comes marrow. Out of marrow comes semen. These are the Sapta Dhatus or the seven Dhatus that support this life and body. Mark here how precious is semen! It is the last essence. It is the Essence of essences. The Veerya comes out of the very marrow that lies concealed inside the bones.
There are three divisions in each Dhatu. Semen retention nourishes the physical body, the heart and the intellect. Only that man who uses the physical body, the heart and the intellect can have perfect Brahmacharya. A wrestler who uses his physical body only, but keeps the intellect and the heart undeveloped, cannot expect to have full Brahmacharya. He can have Brahmacharya of the body only, but not of the mind and the heart. The semen that belongs to the heart and the mind will certainly flow out. If an aspirant does only Japa and meditation, if he does not develop the heart, and if he does not practice physical exercise, he will have only mental Brahmacharya. The portion of the semen, which goes to nourish the heart and the body, will flow out. But an advanced Yogi who dives deep in meditation will have full Brahmacharya even if he does not take physical exercise.
Semen is the quintessence of food or blood. One drop of semen in manufactured out of forty drops of blood according to modern medical science. According to Ayurveda, it is elaborated out of eighty drops of blood. The two testes or seeds that are located in the scrotal bag are called secretory glands. The cells of these testes have been endowed with the peculiar property of secreting semen from the blood. Just as bees collect honey in the honeycomb drop by drop, so also, the cells of the testes collect semen drop by drop from the blood. Then this fluid is taken by the two ducts or tubes to the vesiculae seminalis. Under excitement, it is thrown out by special ducts, called ejaculatory ducts, into the urethra where it is mixed with the prostatic juice.
Semen is found in a subtle state in all the cells of the body. Just as sugar is all-pervading in the sugar-cane, butter in milk, so also, semen is pervading the whole body. Just as the butter milk is thin after the butter is removed, so also, semen is thinned by its wastage. The more the wastage of semen the more is the weakness. In the Yoga Sastras it is said:
Maranam Bindu Patanat,
Jivanam Bindu Rakshanat
It is translated as:
Falling of semen brings death; preservation of semen gives life. Semen is the real vitality in men. It is the hidden treasure in man. It imparts Brahma-Tejas to the face and strength to the intellect.
This concludes the answer given by Swami Sivananda on the topic of semen retention in Yoga and Ayurveda.
The above passage is an excerpt from the book Practice of Bhramcharya by Sivananda Saraswasti.